Not applicable.
Not applicable.
This invention relates generally to communications systems and, more particularly, to planar circulators and methods of fabrication.
As is known in the art, a radar or communications system antenna generally includes a feed circuit and at least one conductive member generally referred to as a reflector or radiator. As is also known, an array antenna can include a plurality of radio frequency (RF) circulators disposed in an array in a manner in which RF signals can be received from or transmitted to the same individual radiator. Sharing the radiators for both transmitting and receiving signals allows a reduction in the size of the antenna in applications where simultaneous transmission and reception is not required. The circulators are also referred to as transmit/receive (T/R) elements.
As is also known in the art, the radio frequency (RF) circulator is a three-port device, having a first, a second, and a third port. A conventional circulator provides a directional capability so that an RF signal applied as an input to the first port provides an output signal at only the second port. Similarly, an RF signal applied as an input to the second port provides an output signal at only the third port, and an RF signal applied as an input to the third port provides an output signal at only the first port.
Conventional circulators are typically provided as discrete devices that can be mounted to a circuit board. Since it contains discrete devices, the conventional circulator does not provide an optimal form factor for high density electronics packaging. In commercial applications, it is often desirable to integrate RF circuits into low profile, low cost packages. For example such devices would be desirable for commercial cell phones. In military surface and airborne applications, there is a need for tile arrays having multiple board layers. Further, in these applications there is a need for low profile, low cost arrays which often require a large number of circulators for corresponding radiators. In conventional systems the circulators are often individually packaged in the transmitter/receiver (T/R) modules thereby increasing module cost and increasing the unit cell footprint so as to reduce an array scan volume versus frequency characteristic due to interference from adjacent lobes in the antenna pattern.
One conventional method (referred to as the discrete method) includes steps for fabricating individual circulators having gaussed (i.e. magnetized) magnets and embedding each individual circulator in a dielectric or metal carrier. This method requires precise alignment and ribbon (or wire) bonding to complete the RF circuit. In addition, the gaussed magnets must be individually magnetized and are exposed to high lamination temperatures during fabrication. Consequently, the magnets experience partial de-magnetization causing a non-uniform magnetization adversely affecting circulator performance. This effect is a function of magnet location across the array. Embedding each individual circulator in a dielectric or metal carrier requires precise individual alignment between the circulator transmission line ports and the carrier transmission line ports. Ribbon (or wire) bonding between circulator transmission lines and board transmission lines to complete an RF circuit requires special plating (e.g., gold plating) for soldering or bonding. Consequently, the RF bandwidth is reduced and signal losses are increased due to process variations that add parasitic reactances to the RF transmission line.
It would, therefore, be desirable to eliminate the ribbon or wire bonding steps, and reduce the alignment tolerances and magnetize (gauss) the magnets after lamination and processing. It would be further desirable to reduce the antenna unit cell spacing by reducing the T/R module footprint to provide a larger scan volume. It would be further desirable to seal the circulators from the environment, and to produce planar assemblies with a plurality of circulators and to produce individual circulators in bulk at a low cost.
In accordance with the present invention, a planar circulator assembly includes a dielectric substrate having a first surface and an opposing second surface, a plurality of circulator circuits each having a first ferrite receiving pad disposed on the first surface and a second ferrite receiving pad disposed on the second surface a first sub-assembly board. The first sub-assembly board is disposed on the first surface, has a plurality of first apertures, a plurality of ferrite-magnet sub-assemblies, each ferrite-magnet sub-assembly disposed in a corresponding first aperture and aligned with a corresponding first ferrite receiving pad and electromagnetically coupled to the corresponding first ferrite receiving pad. The assembly further includes a second sub-assembly board disposed on the second surface having a plurality of second apertures, and a plurality of ferrites each disposed in a corresponding second aperture aligned with a corresponding second ferrite receiving pad and electromagnetically coupled to the corresponding second ferrite receiving pad.
This arrangement eliminates fabrication of individual circulators by embedding each individual circulator in a dielectric or metal carrier. Such an arrangement further eliminates precise alignment and ribbon (or wire) bonding for attaching circulators in fixed orientations to complete the RF circuit by using epoxies and/or solders. With such an arrangement, a plurality of low-profile circulators are embedded in a multi-layer laminate in one bonding step using standard Printed Wiring Board (PWB) and Surface Mount Technology (SMT) processes, for example this arrangement reduces the antenna unit cell spacing by reducing the T/R module footprint in order to provide a larger radar scan volume.
In accordance with a further aspect of the present invention, a planar circulator assembly includes at least one first RF port via disposed in the first sub-assembly board, each first RF port via having a first end coupled to a corresponding one of the first, second and third ports and a second end coupled to a connection disposed on a first outer surface of the circulator assembly. The planar circulator assembly further includes at least one second RF port via disposed in the second sub-assembly board, each second RF via having a first end coupled to one of the first, second and third ports and a second end coupled to a connection disposed on a second outer surface of the circulator assembly disposed opposite the first outer surface. With such an arrangement, the circulators can be bonded to seal the circulators from the environment.
In accordance with a further aspect of the present invention, a method for making an embedded planar circulator assembly includes providing a circulator board having a first surface and an opposing second surface, forming a plurality of circulator circuits disposed on the circulator board, each circuit having a ferrite receiving pad disposed on the first surface and a corresponding ferrite receiving pad on the second surface, providing a plurality of ferrite-magnet sub-assemblies disposed in a first sub-assembly. The method further includes providing a plurality of ferrites disposed in a second sub-assembly, and bonding the circulator board between the first sub-assembly and the second sub-assembly such that the ferrite-magnet sub-assemblies are urged against a corresponding ferrite receiving pad disposed on the first surface of the circulator board and the ferrites are urged against the corresponding ferrite receiving pad on the second surface of the circulator board. With such a technique, the ribbon or wire bonding steps are eliminated, alignment tolerances are reduced and the magnets can be magnetized after the lamination and processing steps.
In accordance with another aspect of the present invention, a method for making an embedded planar circulator assembly further includes separating the plurality of circulator circuits into a corresponding plurality of individual unit cells. With this technique, individual circulators can be produced in bulk in a low profile package and at a low cost.
The relatively high cost of phased arrays has precluded the use of phased arrays in all but the most specialized applications. Assembly and component costs (especially the active transmit/receive module including circulators) are major cost drivers. Phased array costs can be reduced by leveraging batch processing and minimizing touch labor of components and assemblies. In one embodiment, the circulators which are typically discrete components wired into T/R modules, are embedded in Polytetrafluoroethylene (PTFE) dielectric laminates, thus reducing cost and complexity in the T/R modules. In addition, the size of the unit cell of a phased array is reduced by including the array of circulators in a single planar assembly. The embedded planar circulator is fabricated with high temperature bonding adhesives common to the PWB industry and the circulator magnets are conveniently magnetized after bonding. The result is a compact, sealed, low cost and high performance array of circulators in a planar array arrangement. Individual circulators are produced in volume by spacing a plurality of circulators on a single circulator board to facilitate separation into individual unit cells.